The outer most skin layer, i.e. the Stratum Corneum (SC), is perhaps the most versatile biological barrier in the human body. It is an excellent electrical insulator and it prevents the uptake of infectious agents while restricting water loss. The delivery of small amounts of liquids through the SC of humans into the underlying tissue or the sampling of fluids from the underlying tissue is becoming increasingly important in biomedical applications. Microsystem Technology provides means for the fabrication of microscaled liquid transfer needles, i.e. micro needles (micro needle). In the last few years, activity in the micro needle field has been steadily growing. Due to their small dimensions, they can be inserted into the skin painlessly and cause less tissue damage than conventional hypodermic needles. Micro needles have the potential to become the preferred drug delivery device in applications where the transdermal aspect is essential. For example, biotechnology has produced a generation of novel compounds with great therapeutic promise that generally consist of active macromolecules, e.g. proteins. Their oral administration is complicated and the passive diffusion of those compounds across the skin is not a realistic option.
Different hollow out-of-plane micro needle for transdermal applications have been presented before. They are arranged in two dimensional arrays to decrease flow resistance through the device. The array can be achieved with wafer level processing. The openings are at the top of the needle, which increases the risk for clogging. Examples of such needles are disclosed in e.g. U.S. Pat. Nos. 6,132,755 and 6,334,856.
In-plane micro needles have been developed earlier and are characterized by the opening at the shaft of the needle and are less prone to clogging. Such needles are disclosed in e.g. U.S. Pat. Nos. 5,928,207 and 6,375,148. These needles are generally longer than out-of-plane needles. The fabrication of two-dimensional arrays is more difficult to achieve since it cannot be done on wafer level.
Our own group, in collaboration with Datex-Ohmeda (a division of Instrumentarium Corp.), have reported on solid silicon micro needle arrays successfully used for biopotential measurements. See Griss et al in Journal of Microelectromechanical Systems, Volume: 10 Issue: 1, March 2001.
The mechanical strength of those micro needle arrays was observed to be surprisingly high, in particular during measurements of the activity of the brain where the arrays were applied on the forehead of test subjects. The mechanical strength of barbed micro needle was also observed when measuring the attachment force of their arrays pressed into different types of materials. Very low failure rate is a requirement for a micromachined micro needle device to be used in commercial applications. In the case of hollow micro needle designed for transdermal liquid transfer, they must be robust enough to penetrate biological tissue and withstand harsh treatment. Coating in-plane single crystalline silicon micro needle with Parylene provides a way to prevent catastrophic failure. This allowed the retraction of micro needle from pierced gelatin membranes, even if the silicon core is fractured. Two dimensional needle arrays are less prone to fracturing when exposed to shear forces during penetration than single needles of the same material and dimensions since the shear stress created by the tissue is distributed over a large amount of micro needle.